1. Field of the Invention
The present invention relates to a torque distribution control device for a four-wheel drive vehicle and more particularly, to a control function for continuing the four-wheel drive operation to secure the drive-through capability of the vehicle in the event that an abnormality occurs.
2. Discussion of the Related Art
Heretofore, a torque distribution control device for a four-wheel drive vehicle has been known, in which a torque distribution device is provided for distributing the drive power transmitted from an engine to prime drive wheels, to sub-drive wheels. In the control device, an abnormality may happen with the wheel speed signals. The cause of such an abnormality may be the fault of any of wheel speed sensors for four front and rear wheels, the breaking or short circuit of signal wires for the wheel speed sensors or the like.
In the event of the abnormality occurring, the torque distribution control device usually switches over a command value to the torque distribution device to zero (0), thereby diminishing the distribution torque to the sub-drive wheels to zero (0). That is, in the event of an abnormality occurring, the control device changes the drive mode from four-wheel drive to two-wheel drive. This comes from the conclusion that continuing four-wheel drive control based on the wheel speed signals which are in short of reliability could influence on the stability in steering the vehicle.
However, where changing the drive mode to the two-wheel drive is carried out in the event of an abnormality occurring as mentioned above, it may occur that the vehicle in the two-wheel drive mode cannot run in such a situation that the four-wheel drive is needed like, for example, a heavily snowy road. This disadvantageously makes it impossible to secure a drive-through capability of the vehicle.
Further, where an abnormality occurs with the wheel speed signal for one wheel of either the front wheels or the rear wheels, it has been practice that the command torque to the torque distribution device is reduced to zero thereby to drive the prime drive wheels only. As a result, when the abnormality arises on the wheel speed signal for only one wheel of those signals for the four wheels, the vehicle is caused to operate in the two-wheel drive mode. Thus, when the abnormality arises on the wheel speed signal for one wheel with the vehicle running on a slippery road such as snowy road, the four-wheel drive vehicle is operated as if it is a two-wheel drive vehicle, whereby it becomes easier for the vehicle to slip.
Furthermore, a so-called tight-cornering phenomenon is known as the phenomenon which occurs on a four-wheel drive vehicle when it runs a small-radius corner while the rotational difference between the front and rear wheels is restrained. In the phenomenon, the vehicle is brought into the state that brake forces are applied to the front and rear wheels. In particular, the tight-cornering phenomenon causes the four-wheel drive vehicle to be liable to suffer the engine stall while the four-wheel drive vehicle turns along a small-radius corner.
To obviate this problem, in a torque distribution device for a four-wheel drive vehicle, the wheel speeds of the four front and rear wheels are detected, and the steering angle is calculated, the rotational speed difference between the front wheels and the rear wheels is detected, or the slip of the wheels located outside with respect to the turning center are calculated based on the wheel speed data. And, when the steering angle is smaller than a predetermined angle, when the rotational speed difference is larger than a predetermined value, or when the wheels located outside with respect to the turning center suffer slip, it is judged that the tight-cornering phenomenon is to occur, and the transmission torque to sub-drive wheels is controlled to zero or to a minimum value in order to prevent the tight-cornering phenomenon from occurring.
However, it may happen that an abnormality arises on wheel speed sensors or signal wires therefor, and the wheel speed data indicates wrong values in the abnormal state. This may allow the judgment of the tight-cornering phenomenon to be performed based on the wrong wheel speed data. Thus, the judgment of the tight-cornering phenomenon becomes inaccurate or unreliable, and hunting takes place in the torque transmission to the sub-drive wheels. This disadvantageously may result in making the movement of the vehicle unstable.